A diesel engine (also known as a compression-ignition engine) is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber. This is in contrast to spark-ignition engines such as a petrol or gasoline engine or gas engine (that uses a gaseous fuel as opposed to gasoline), which uses a spark plug to ignite an air-fuel mixture.
The diesel engine has the highest thermal efficiency of any regular internal or external combustion engine due to its very high compression ratio. Low-speed Diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) often have a thermal efficiency which exceeds 50 percent.
Diesel engines are manufactured in two stroke and four stroke versions. They have been used in submarines and ships, locomotives, trucks, heavy equipment and electric generating plants, and eventually in automobiles, on-road and off-road vehicles.
The diesel internal combustion engine differs from the gasoline powered Otto cycle by using highly compressed hot air to ignite the fuel rather than using a spark plug (compression ignition rather than spark ignition). In the true diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 22:1 resulting in 40-bar (4.0 MPa; 580 psi) pressure compared to 8 to 14 bars (0.80 to 1.4 MPa) (about 200 psi) in the gasoline/petrol engine. This high compression heats the air to 550° C. (1,022° F.). At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. This may be into a (typically toroidal) void in the top of the piston or a pre-chamber depending upon the design of the engine. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of the compressed air vaporizes fuel from the surface of the droplets. The vapor is then ignited by the heat from the compressed air in the combustion chamber, the droplets continue to vaporize from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. The start of vaporization causes a delay period during ignition and the characteristic diesel “knocking” sound as the vapor reaches ignition temperature and causes an abrupt increase in pressure above the piston. The rapid expansion of combustion gases then drives the piston downward, supplying power to the crankshaft.
As well as the high level of compression allowing combustion to take place without a separate ignition system, a high compression ratio greatly increases the engine's efficiency. Increasing the compression ratio in a spark-ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent damaging pre-ignition. In a true Diesel engine, premature detonation is not an issue because only air is compressed in a diesel engine, and fuel is not introduced into the cylinder until shortly before top dead centre (TDC), and compression ratios are much higher.
Diesel engines in service today raise the fuel to extreme pressures by mechanical pumps and deliver it to the combustion chamber by pressure-activated injectors without compressed air. With direct injected diesels, injectors spray fuel through 4 to 12 small orifices in its nozzle. The early air injection diesels always had a superior combustion without the sharp increase in pressure during combustion. Air injection-aided spraying can improve dispersion and reduce droplet size.
Diesel engines employ a mechanical or electronic governor that regulates the idling speed and maximum speed of the engine by controlling the rate of fuel delivery. Unlike Otto-cycle engines, incoming air is not throttled and a diesel engine without a governor cannot have a stable idling speed and can easily overspeed, resulting in its destruction. Mechanically-governed fuel injection systems are driven by the engine's gear train. These systems use a combination of springs and weights to control fuel delivery relative to both load and speed. Modern electronically controlled diesel engines control fuel delivery by use of an electronic control module (ECM) or electronic control unit (ECU). The ECM/ECU receives an engine speed signal, as well as other operating parameters such as intake manifold pressure and fuel temperature, from a sensor and controls the amount of fuel and start of injection timing through actuators to maximize power and efficiency and minimize emissions. Controlling the timing of the start of injection (SOI) of fuel into the cylinder can minimize emissions, and improve fuel economy (efficiency), of the engine. The timing is measured in degrees of crank angle of the piston before top dead center. For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection, or timing, is said to be 10° BTDC. Optimal timing will depend on the engine design as well as its speed and load.
Advancing the start of injection (injecting before the piston reaches to its SOI-TDC) results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in elevated engine noise and increased oxides of nitrogen (NOx) emissions due to higher combustion temperatures. Delaying start of injection causes incomplete combustion, reduced fuel efficiency and an increase in exhaust smoke, containing a considerable amount of particulate matter and unburned hydrocarbons.
Present day diesel engines use a camshaft—(rotating at half crankshaft speed) lifted, mechanical single plunger with a high pressure fuel pump (driven by the engine crankshaft). For each cylinder, the plunger measures the amount of fuel and determines the timing of each injection. These engines use injectors that are very precise spring-loaded valves that open and close at a specific fuel pressure. For each cylinder a plunger pump is connected to an injector with a high pressure fuel line. Fuel volume for each single combustion is controlled by a slanted groove in the plunger which rotates only a few degrees releasing the pressure, and is controlled by a mechanical governor, consisting of weights rotating at engine speed constrained by springs and a lever. The injectors are held open by the fuel pressure. On high speed engines the plunger pumps are together in one unit. Each fuel line should have the same length to obtain the same pressure delay.
A less complex configuration on high speed engines with fewer than six cylinders is to use an axial-piston distributor pump, consisting of one rotating pump plunger delivering fuel to a valve and line for each cylinder (functionally analogous to points and distributor cap on an gasoline engine). Another method uses a single fuel pump which supplies fuel to each injector constantly at high pressure with a common rail (single fuel line common). Each injector has a solenoid operated by an electronic control unit, resulting in more accurate control of injector opening times that depend on other control conditions, such as engine speed and loading, and providing better engine performance and fuel economy. This design is also mechanically simpler than the combined pump and valve design, making it generally more reliable, and less loud, than its mechanical counterpart.
Modern diesel engines make use direct injection methods. One type is a direct injection injector mounted in the top of the combustion chamber, with electronic control of the injection timing, fuel quantity, EGR and turbo boost, giving more precise control of these parameters which eased refinement and lowered emissions. Unit direct injection injects fuel directly into the cylinder of the engine, combining the injector and the pump into one unit positioned over each cylinder controlled by the camshaft. Each cylinder has its own unit eliminating the high pressure fuel lines, achieving a more consistent injection.
In a two-stroke diesel engine, as the cylinder's piston approaches the bottom dead center, exhaust ports or valves are opened, relieving most of the excess pressure after which a passage between the inlet air box and the cylinder is opened, permitting air flow into the cylinder. The air flow blows the remaining combustion gases from the cylinder—this is the scavenging process. As the piston passes through bottom center and starts upward, the passage is closed and compression commences, culminating in fuel injection and ignition.
Diesels are now turbocharged, and some are both turbo charged and supercharged. Because diesels do not have fuel in the cylinder before combustion is initiated, more than one bar (100 kPa) of air can be loaded in the cylinder without preignition. A turbocharged engine can produce significantly more power than a naturally aspirated engine of the same configuration, as having more air in the cylinders allows more fuel to be burned and thus more power to be produced. A supercharger is powered mechanically by the engine's crankshaft, while a turbocharger is powered by the engine exhaust, not requiring any mechanical power. Turbocharging can improve the fuel economy of diesel engines by recovering waste heat from the exhaust, increasing the excess air factor, and increasing the ratio of engine output to friction losses.
A two-stroke engine does not have a discrete exhaust and intake stroke and thus is incapable of self-aspiration. Therefore all two-stroke engines must be fitted with a blower to charge the cylinders with air and assist in dispersing exhaust gases, a process referred to as scavenging. In some cases, the engine may also be fitted with a turbocharger, whose output is directed into the blower inlet. A few designs employ a hybrid turbocharger for scavenging and charging the cylinders, which device is mechanically driven at cranking and low speeds to act as a blower.
As turbocharged or supercharged engines produce more power for a given engine size as compared to naturally aspirated engines, attention must be paid to the mechanical design of components, lubrication, and cooling to handle the power. Pistons are usually cooled with lubrication oil sprayed on the bottom of the piston. Large engines may use water, sea water, or oil supplied through telescoping pipes attached to the crosshead.
A stratified charge engine is a type of internal-combustion engine, similar in some ways to the Diesel cycle, but running on normal gasoline. The name refers to the layering of fuel/air mixture charge inside the cylinder. In a traditional Otto cycle engine, the fuel and air are mixed outside the cylinder and the mixture is drawn into the cylinder during the intake stroke. The air/fuel ratio is kept very close to stoichiometric, which is defined as the exact amount of air necessary for a complete combustion of the fuel. This mixture is easily ignited and burns smoothly. The problem with this design is that after the combustion process is complete, the resulting exhaust stream contains a considerable amount of free single atoms of oxygen and nitrogen, the result of the heat of combustion splitting the O2 and N2 molecules in the air. These will readily react with each other to create nitrous oxide (NOx), a pollutant. A catalytic converter in the exhaust system re-combines the NOx back into O2 and N2 in modern vehicles
A direct injection diesel engine, on the other hand, injects diesel fuel (which is heavier and resistant to vaporization) directly into the cylinder, the combustion chamber is in the top of the piston. This has the advantage of avoiding premature spontaneous combustion—a problem known as detonation or ping that plagues the Otto cycle engines when the fuel-air mixture pre-detonates with high compression conditions—and allows the diesel to run at much higher compression ratios. This leads to a more fuel-efficient engine, which is commonly found in applications where it is being run for long periods of time, such as in trucks and industrial power plants.
However the Diesel engine has problems as well. The fuel is sprayed right into the highly compressed air and has little time to mix properly. This leads to portions of the charge remaining almost entirely air and other portions almost entirely of unburnt fuel lacking for oxygen. This incomplete combustion leads to the presence of other pollutants such as partially burnt and unburnt fuel-polycyclic aromatic hydrocarbons and the plainly visible exhaust soot. The indirect injection diesel where fuel is injected into a pre-chamber (the best known being Ricardo Consulting Engineers' Ricardo Comet design), where the flame front from the pre-chamber ignition leads to better mixing of the air and fuel, smoother combustion in the cylinder, and a reduction in diesel knock. Indirect injection diesels are a kind of stratified charge engine. These benefits came at the cost of a 10% efficiency reduction compared to direct injection diesels.
The stratified charge design attempts to fix the problems with both fuels. It uses a direct-injection system, like the diesel, with its inherent ability to be run at efficient high compressions. However, like the Otto, the stratified charge design relies on gasoline's ability to mix quickly and cleanly in order to avoid the poor combustion found in older direct injection diesels. To do this the fuel injectors are aimed to inject the fuel into only one area of the cylinder, often a small “subcylinder” at the top, or periphery, of the main cylinder, to provide. a rich charge in that area that ignites easily and burns quickly and smoothly. The combustion process proceeds and moves to a very lean area (often only air) where the flame-front cools rapidly and the harmful NOx has little opportunity to foam. The additional oxygen in the lean charge also combines with any CO to form CO2, which is less harmful. This technology has also been applied to the latest electronically controlled direct injection diesels. The injection system on these engines delivers the fuel in multiple injection bursts to ensure better fuel/air mixing and reduced diesel knock. The much cleaner combustion in stratified charge gasoline engines allows for the elimination of the catalytic converter and allows the engine to be run at leaner (lower ratio of fuel to air) mixtures, using less fuel. It has had a similar effect on diesel engine performance. Today's diesels are cleaner and can be twice as powerful as before, while maintaining similar fuel economy.
After years of trying, this layout has proven not to be terribly easy to arrange. The system has been used for many years in slow-running industrial applications, but has generally failed to develop into an automobile engine. Many attempts have been made over the years, notably in Wankel engine applications, but only the Japanese car manufacturers have pressed ahead with piston-engine development.
There remains an important need to provide a low cost, high horsepower internal combustion engine having improved fuel efficiency and low NOx and particulate emissions, that can operate with a variety of fuels.